That is a serious concern. You are putting the money away tax free. It grows. No one knows at what rate it will be taxed at when you begin to take it out. The Roth IRA is much better. However, it is set up so you really can't put in large enough sums to make a difference. They will take as much as they can from those of us that work.
Just as no one knows at what rate it the 401k be taxed at when you begin to take it out, I can see the govt. down the road say there is a 'crisis' with SS, and Roth IRA's are no longer tax free.